Larry Van Wormer's LX5 Modifications

LX5 Modifications

After hearing a demo of these speakers, I recommended them to
a friend, as a good-for-the-money alternative for his stereo system. Nice highs, although
no real bass. While I didnt really like the mid-range that much, I was confident I
could get that working right. So, he got them, and after some breakin turned them over to
me for modification.

Since then, I have thoroughly tested the speakers, using
Liberty Audiosuite, and have completely dismantled them, to check out their construction.
To summarize whats needed, in approximate order of importance:

the crossover needs a complete rework, to eliminate or at least
minimize several problems

cabinet stuffing needs improvement, to eliminate internal
resonances

some mods to reduce the spl peak at about 100 Hz.

efforts to minimize diffraction problems with the tweeter
location are worthwhile, though not near as important as the first three items.

stiffening of the cabinet walls helps a little bit to clear
things up.

Details of the Problems and the Modifications

Drivers:

Tweeter: this dipole has a fairly odd dispersion pattern.
Straight ahead, there is a deep "suckout" in the response, centred at about 16
kHz which is 30 db down! Its very narrow in range, however, and even at 7 degrees
off-axis, is mostly gone. In sum, the tweeter output is fairly smooth and stable up to
about 10 kHz, above that it varies significantly with listening angle. (Its output
is about the same, on average, as the woofer, so some of the usual crossover options to
smooth the output cant be used.)

Woofer: pretty average, below about 3000 Hz. Above that, the
response falls off rapidly, then rises again steeply, likely due to cone breakup
(waterfall plots show a strong resonance centered around 5 kHz.)

Total amplitude of the dip and peak is more than 24 db!

Not much bass, but thats no surprise. There is a
significant peak in output at about 100 Hz, since the box is really too small for this
driver. Some effort to reduce that is worthwhile.

Crossover:

The stock crossover doesnt do much to control the driver
problems. Its nominally first order on both drivers: a 1 mH coil on the woofer, and
a 6.2 uF cap on the tweeter. What with that peak in the woofer response, its just as
loud as the tweeter at 5000 Hz. Since the drivers arent even nearly in phase, the
result isnt too impressive.

So, having thoroughly tested the drivers, and developed the
Calsod models, I started trying various options. A quick examination of the test results
convinced me there was no way to use my favourite first-order crossover designs: the
drivers just dont have the operating range needed, and in any case are not
time-aligned.

I worked for a bit in trying to smooth out the tweeter, but
soon gave up. Its dispersion pattern above 10kHz is just too irregular to do much
about. Below 10 kHz, however, its quite stable, so I decided to just work on a
smooth transition with the woofer.

I decided to go as far as practical in the direction of steep
rolloffs, to reduce the crossover range. A fair bit of Calsod simulation demonstrated that
a sixth order crossover at about 2000 Hz was quite practical, by taking advantage of the
driver rolloffs. Following my usual practice, I developed a crossover by adding the
various needed functions one by one, then adjusting and optimizing to simplify. The result
is quite simple, and meets the following criteria:

smooth response, except for the variable tweeter dispersion.

minimal overlap of the driver operating ranges (while the
woofer peak at 5000 Hz still exists, it is more than 30 db down from reference, which is
not troublesome) Both driver rolloffs are very close indeed to true sixth order, so the
crossover range is quite narrow.

good phase matching between the drivers, in the crossover
range. (Not perfect, but pretty good)

good tweeter protection

reasonable impedance characteristics. The impedance drops to
about 4 ohms at one spot, is normally around 5 ohms, and has one 15 ohm peak.

(If anyone wants to use other types of components, of course,
go ahead.)

C1 can be the original component, with a 1 uF polypropylene
bypass

C2 should be at least a cheap polypropylene, such as Solen or
Bennic

L1 can be a cheap aircore, such as Madisounds cheapest
line.

L2 and L3 should be aircores, at least equivalent to
Madisounds Sidewinder series

C3 and C4 can be something cheap, such as Bennic bipolars

R1 can be a cheap power resistor, 15 watts is plenty.

For the internal layout, I just separated the coils as much as
practical, and arranged them so their axes were at right angles. All crossover components
glued into the cabinet. For internal wiring, I used some teflon-insulated wire I had
available, Id guess about 16 gauge. Im sure anything above 20 gauge or so
should be fine.

The tweeter dispersion above 10000 Hz is not even, but it
actually sounds pretty good, I suspect because of its dipole characteristic.
Its putting more energy into the room than a normal tweeter would, at a given spl.
Its worth playing around with speaker room position and toe-in. Based on my
listening trials to date, I would aim to have the speakers at least 18" from any
wall.

Cabinet:

The woofer cabinet is cast aluminum, with a stiff plastic
front. Its structure is fairly good, but the aluminum does tend to ring. I simply
cross-braced it at a couple of points, with 1/2" dowel, glued in place with
"Liquid Nails" adhesive.

Cabinet stuffing was worth more attention, to get rid of the
backwave, while maintaining the basic box resonance, for some semblance of bass. I used
pieces of 1/4" thick felt to divide the cabinet up into sections that had
non-parallel sides. Many possible variations exist: the basic idea is to eliminate the
parallel wall resonance problem, by positioning felt at angles across the cabinet. Of
course, you must not seal off any portion of the cabinet completely. More Liquid Nails to
hold the felt in place.

Since my friend wanted to use the speakers alone, without a
sub, I tried to maintain some bass, while reducing the 100 Hz peak as much as possible. I
liked the effects of plugging the shorter of the two speaker ports, and leaving the other
operational.

If you plan to use the speakers in combination with a sub,
then by all means plug both ports, and stuff the box with polyester batting or similar.
That will reduce the 100 Hz peak by several db, at the cost of any bass below 80 Hz or so.

The top tweeter location results in several diffraction
problems: my friend insisted on maintaining the stock cage arrangement, but I was able to
test out several variations. Felt on top of the cabinet helped a bit, but high density
felt is needed to get a significant reduction in the reflected sound. (BTW, Libertys
impulse measurements are *very* nice for testing the various options!)

The final result: very nice! A very clear midrange, and the
dipole tweeter has a very "open" sound to it. Overall, well worth the cost and
effort.